1. Field of the Invention
The present invention relates to methods for preparing aqueous dispersions comprising alkyl (meth)acrylates which are suitable as pressure sensitive adhesives.
2. Description of the Background Art
Pressure sensitive adhesives (also known as "self-adhesive compounds") form a permanently adhesive film which, under slight pressure, adheres immediately to various surfaces at room temperature. (See 1985, "Ullmann's Encyclopedia of Industrial Chemistry", 5th Ed., Vol. A1, pub. VCH, pp. 235-236.)
Pressure sensitive adhesives are used principally for coating of supports, such as those based on paper or polymer films or sheets. These coated supports provide products such as self-stick labels, adhesive tapes and self-stick film which have a characteristic broad spectrum of properties, including:
short-contact-time adhesion of the adhesive film to the substrate surface ("tack") and PA1 water-resistance of the adhesive layer. PA1 "There are indications that a pressure sensitive adhesive must always be comprised of PA1 a high polymer principal resin, which contributes cohesion and the specific adhesion, and PA1 so-called tackifiers,
Understandably, a single pressure sensitive adhesive cannot display all desirable properties, particularly those properties which are mutually exclusive. Accordingly, a compromise must be made and an "optimized" set of properties must be found for each application. Often, product optimization is accomplished by the use of various auxiliary substances, such as synthetic resins, tackifiers, cross-linking agents and viscosity regulators. The particular adhesives, depending on their compositions, are applied to the substrates in the form of solutions, dispersions, or even melts. After the adhesive sets, the layer of pressure sensitive adhesive is present as a film on the support. Prior to use, the film is often covered, with a suitable anti-adhesive material, such as a silicone-based film. However, adhesive tapes may also be employed with no covering layer (see "Ullmann's Encyclopedia", loc.cit., 258). Natural and synthetic rubbers, in modified form, may be used as adhesive materials for pressure sensitive adhesives. Also frequently used are adhesives prepared from poly(meth)acrylic acid esters, polyvinyl ethers, and polyisobutylene, often in combination with phenolformaldehyde- or hydrocarbon resins.
Polyacrylate dispersions or vinyl acetate copolymers are also often used as so-called "dispersion pressure sensitive adhesives" to which resins are often added
The problem with most pressure sensitive adhesives can be better understood by way of an illustrative example based on adhesive tapes made of (corona treated) polypropylene strip material. Such adhesive tapes are often used as packaging tape for cardboard boxes. The most important requirements in this area of application are for good adhesion of the tape to the surface of the box, and good internal strength (or cohesion) of the adhesive layer. In most cases, it has not been possible to satisfy both of these requirements simultaneously, because many of the means of producing good cohesion are detrimental to good adhesion. For example, while it is known that cohesion increases with increasing molecular weight of the polymer, adhesion decreases correspondingly.
In Ullmann, loc.cit., it is stated:
wherewith in many systems the latter may be replaced by low molecular weight components of the principal polymer. To increase cohesion, in many systems the principal resin is crosslinked (or vulcanized, in the case of rubbers) after application."
Certain techniques for preparing the adhesive by emulsion polymerization have found favor, despite the fact that a number of variants are known. Thus, the use of ammonium peroxydisulfate (APS) as an initiator has become standard procedure. Conventionally peroxydisulfate compounds are used as initiators in amounts of 0.001-0.05 wt. % (based on the weight of the monomers) (see Houben and Weyl, 1961, "Methoden der Organischen Chemie", 4th Ed. Vol. XIV/1, pub. G. Thieme, p. 1049; and 1968, Rauch-Puntigam, H. and Voelker, Th., "Acryl- und Methacrylvergindungen", pub. Springer-Verlag, 221).
Thus, it is desired in the art to improve the overall set of properties of the known acrylate pressure sensitive adhesives, particularly their adhesion and cohesion, without requiring the use of auxiliary additives as noted above.